The Owls are set to be backed with funds in the summer, but Lees insists that they must not focus all their attention on incoming players.
The former Leeds United man feels that Wednesday have the bulk of a decent squad already in place and he wants the club to ensure that none of their key stars depart Hillsborough in the coming months.
“The summer for us is about improving and moving forward," he said. "It’s crucial we do not lose people and replace them with not as good players.
“We need to keep our best players and add to what we have got. If we can do that, hopefully we can take a step forward.”
Wednesday are likely to be amongst the promotion favourites next season and Lees will be one of their main players.
The centre-back has been in fine form since arriving at Wednesday from Leeds and he has formed part of one the strongest back-lines in the Championship.
The Owls have been a tough outfit to break down all season, but their main priority in the summer will be to improve their strike-force.
Stuart Gray will realise that he needs to add more goals to his side and making sure he recruits the right striker will be key to their chances of meeting expectations next term.